Method of preparing granulated sugar from an aqueous sugar solution containing monovalent and polyvalent anions and cations

ABSTRACT

A purification method employs nanofiltration of an aqueous solution containing one or several sugars, multivalent cations, monovalent metal cations, monovalent anions and multivalent inorganic anions and/or organic acid anions. The method includes replacement of at least a part of said multivalent cations and/or said multivalent inorganic anions and organic acid anions respectively by monovalent metal cations and/or monovalent anions to produce a solution. Nanofiltration of the solution is carried out to obtain a retentate, and at least part of the retentate is subject to crystallization.

This application is a 371 of PCT/FR03/02593, filed 27 Aug. 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a preparation method of crystallizedsugar from an aqueous solution containing one or several sugars,multivalent cations, monovalent metal cations, monovalent anions andmultivalent inorganic anions and/or organic acid anions, with priorpurification of this aqueous solution by nanofiltration.

2. Description of the Related Art

The purification by demineralization of liquid products (such as aglucose syrup, sweet juices or whey) by means of ion-exchange resins hasbeen known for many years.

The principle of such a demineralization is to make such liquid productpercolate through a cationic resin and an anionic resin, the counter-ionof the former being the H⁺ ion and the counter-ion of the latter, theOH⁻ ion.

While passing through the cationic resin, the cations of the liquidproduct are exchanged with the H⁺ ions of the resin, and while passingthrough the anionic resin, the anions of the product are exchanged withthe OH⁻ ions of this resin, the H⁺ and OH⁻ ions thus released from saidresins combining together to give water.

The regeneration of the resins thus used is performed by passage of anacid on the cationic resin and of a base on the anionic resin, andaccording to the regeneration outputs, the regeneration effluents cancontain up to 2 to 3 times the inorganic load extracted from the treatedliquid product.

Such highly saline effluents constitute indisputably a source ofpollution.

In short, the demineralization of liquids containing inorganics by meansof ion-exchange resins calls for the use of an acid and of a base fortheir regeneration. Besides the cost of these chemicals, thisdemineralization procedure produces pollutant saline effluents, and ofwhich the processing is onerous.

Moreover, the nanofiltration technique is generally used as apre-concentration means of aqueous solutions containing inorganics. Themonovalent ions of these inorganics migrate through the nanofiltrationmembrane and therefore most of them are found in the permeate, whereastheir multivalent ions are retained by this membrane and most of themare found concentrated in the retentate; a purification effect bydemineralization is thus obtained, which remains however insufficient.

SUMMARY OF THE INVENTION

The aim of the present invention is the development of a preparationmethod of crystallized sugar comprising a prior purification, economicalin energy and in chemicals and limiting the quantity and the number ofproduced effluents.

The basic idea of this method lies in the modification of the ioniccomposition, without demineralization, of the aqueous solution to betreated in order to improve the demineralization effect of ananofiltration carried out on the aqueous solution thus modified.

Thus, the present invention relates to a preparation method ofcrystallized sugar from an aqueous solution containing one or severalsugars, multivalent cations, monovalent metal cations, monovalent anionsand multivalent inorganic anions and/or organic acid anions, such aslactate or citrate, characterized in that it comprises the operations:

-   -   (a) of replacing at least a part of said multivalent cations        and/or of said multivalent inorganic anions and organic acid        anions by monovalent metal cations and/or monovalent anions,        respectively, in order to obtain an aqueous solution depleted in        multivalent cations and/or multivalent inorganic anions and        organic acid anions, and containing said monovalent metal        cations and/or said monovalent anions, and    -   (b) of nanofiltration of the solution resulting from        operation (a) in order to obtain as a retentate, a sugar aqueous        juice enriched in sugars, in multivalent cations and in        multivalent inorganic anions and/or in organic acid anions, and        as a permeate, an aqueous effluent containing most of the        monovalent anions and monovalent metal cations,    -   (c) of crystallization of at least a part of the retentate        resulting from operation (b), in order to obtain crystallized        sugar and a mother liquor enriched in monovalent anions and        monovalent metal cations.

Operation (a) above provides an aqueous solution enriched in monovalentanions and/or monovalent metal cations and greatly depleted inmultivalent cations and in multivalent inorganic anions and/or organicacid anions.

During operation (b), the sugars of the aqueous solution resulting fromoperation (a) are found in the retentate in which are also principallyfound the remaining multivalent cations and multivalent inorganic anionsand/or organic acid anions. As for the monovalent ions, most of them arefound in the permeate.

It is to be noted that thanks to prior operation (a), which does notconstitute in itself a demineralization operation, the proportion of themonovalent ions relatively to the multivalent ions and organic acidanions is increased in the aqueous solution, which causes an increase ofthe demineralization rate of said aqueous solution during operation (b).

When one tries to preferably eliminate the multivalent cations presentin the aqueous solution to be purified, in operation (a) the replacementof the multivalent cations is advantageously performed simultaneously tothe replacement of the multivalent inorganic anions and/or organic acidanions, or still more advantageously performed on the aqueous solutionhaving beforehand undergone the replacement of the multivalent inorganicanions and/or organic acid anions.

Moreover, when one tries to preferably eliminate the multivalentinorganic anions and/or organic acid anions present in the aqueoussolution to be purified, in operation (a) the replacement of themultivalent inorganic anions and/or organic acid anions isadvantageously performed simultaneously to the replacement of themultivalent cations or still more advantageously performed on theaqueous solution having beforehand undergone the replacement of themultivalent cations.

According to a preferred embodiment of the invention, replacementoperation (a) comprises the processing of the aqueous solution with acationic resin of which the counter-ion is a monovalent metal cationand/or with an anionic resin of which the counter-ion is a monovalentanion.

Moreover, the monovalent metal cation forming the counter-ion of thecationic resin and the monovalent anion forming the counter-ion of theanionic resin are preferably of the same type as said monovalent metalcations and said monovalent anions, respectively present in the initialaqueous solution; this prevents the introduction of foreign ions in theprocess and makes more advantageous, as it will be seen below, theregeneration operations of the aforementioned cationic and anionicresins.

According to an important characteristic of the present invention, thismethod preferably further comprises an operation:

-   -   (d) of regeneration of the cationic and/or anionic resins,        particularly by processing the same by a permeate obtained        during nanofiltration operation (b) above, this permeate being        concentrated beforehand at the desired degree.

In proceeding that way, we make use, for the regeneration, of themonovalent ions initially present in the aqueous solution to bepurified; this prevents the use of costly chemicals foreign to themethod and limits the production of polluting effluents.

According to various alternatives, the method according to the inventioncan further comprise one or several of the following operations:

-   -   (e) chromatography of at least a part of the mother liquor        produced by crystallization operation (c), in order to obtain an        effluent enriched in sugar and a raffinate enriched in        monovalent anions and monovalent metal cations;    -   (f) chromatography of a part of the retentate resulting from        operation (b), in order to obtain an effluent enriched in sugar        and a raffinate enriched in monovalent anions and monovalent        metal cations; and    -   (g) processing of the permeate resulting from operation (b), by        reverse osmosis or electrodialysis in order to produce water and        an aqueous fraction enriched in monovalent anions and monovalent        metal cations.

It will be noted that according to another characteristic of the methodof the present invention, the cationic resin and/or anionic resin can beregenerated by processing it/them with at least one of the followingliquids, possibly concentrated, combined to a part of the permeateobtained during operation (b): mother liquor obtained during operation(c), raffinate obtained during operation (e), raffinate obtained duringoperation (f), aqueous fraction obtained during operation (g).

The method according to the invention can be used in particular for thepurification of a whey, of a permeate resulting from the ultrafiltrationof a whey or of a juice of sugar beetroot, sugarcane, chicory orJerusalem Artichokes, this whey, permeate or juice comprising Ca²⁺ andMg²⁺ ions, Cl⁻ anions, Na⁺ or K⁺ cations and anions selected mainly inthe group consisting of phosphate and sulfate anions, anions fromorganic acids and their mixtures.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is illustrated hereafter, in a non limitativemanner, by the description of a purification example, done withreference to the unique FIGURE which is the schematic representation ofan installation for the carry out of the method according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The aqueous solution subject to this method is in the selected example apermeate obtained by ultrafiltration of a whey. Such a permeatecomprises mainly lactose, organic acids and inorganics (particularlyNa⁺, K⁺, Ca²⁺, Mg²⁺ cations, Cl⁻ and phosphate anions and organic acidanions, such as citrate and lactate).

This permeate is carried by a duct 1 to the entrance of a column 2filled with a strong anionic resin (AF), then from the exit of thiscolumn 2 by a duct 3 to the entrance of a column 4 filled with a strongcationic resin (CF).

The strong cationic resin is in the Na⁺ or K⁺ form, i.e. its counter-ionis the Na⁺ or K⁺ ion; the strong anionic resin is in the Cl⁻ form, i.e.its counter-ion is the Cl⁻ ion.

It will be noted that, as an alternative, both these resins could beused in a mixture, in which case a single column would be sufficient.

During the migration of the permeate through the anionic resin, itexchanges its multivalent inorganic anions (phosphate) and organic acidanions (lactate, citrate) with the Cl⁻ ions of the resin; during itsmigration through the cationic resin, it exchanges its multivalentcations (Ca²⁺, Mg²⁺) with the Na⁺ or K⁺ ions of the resin.

The permeate is therefore relieved from a substantial part of itsmultivalent cations and multivalent inorganic anions and of its organicacid anions, which cations and anions have been replaced by monovalentcations and anions; this permeate therefore mainly contains lactose,Na⁺, K⁺ and Cl⁻ ions, residual Ca²⁺, Mg²⁺ cations, residual phosphateanions and residual organic acid anions.

The aqueous solution coming from column 4 is then carried by a duct 5 ina nanofiltration device 6 comprising one or some nanofiltrationmembranes permeable to the monovalent ions, but retaining the lactose,the multivalent ions and the organic acid ions.

Thus, are coming from device 6:

on one hand, by duct 7, a permeate enriched in Cl⁻, Na⁺ and K⁺ ions, and

on the other hand, by duct 8, a retentate enriched in lactose and inphosphate anions, anions from residual organic acids and residual Ca²⁺and Mg²⁺ cations; this retentate further contains a small quantity ofNa⁺, K⁺ and Cl⁻ ions.

Moreover, a part of the nanofiltration retentate is subject to acrystallization. For this purpose, a branching 11 is provided on duct 8,this branching 11 leading to a crystallization unit 12 where areproduced crystallized lactose and a mother liquor containing most of theminerals (particularly Na⁺, K⁺ and Cl⁻).

If desired, at least a part of this mother liquor can be subject to achromatography in a chromatography column 13, in order to obtain on onehand, an effluent enriched in lactose and a raffinate enriched inminerals (mainly Na⁺, K⁺ and Cl⁻).

Further, another part of the nanofiltration retentate can be subject toa chromatography. For this purpose, a second branching 14 is provided onduct 8, this branching leading to the entrance of a chromatographycolumn 15. Is extracted from the latter, on one hand, an effluentenriched in lactose, and on the other hand a raffinate enriched ininorganics (mainly Na⁺, K⁺ and Cl⁻).

It will be noted that the nanofiltration permeate removed through duct7, can be treated in a reverse osmosis unit 22 in order to obtain, onone hand, water extracted by duct 23, and on the other hand, an aqueousfraction (removed through duct 24) concentrated in Na⁺, K⁺ and Cl⁻ ions.

As it is evident from the foregoing, we have a whole range of liquidsproduced during the method and advantageously usable, if necessary afterconcentration, for the regeneration of the strong cationic resin and ofthe strong anionic resin filling columns 2 and 4.

That is:

a fraction of the nanofiltration permeate removed through duct 7,

the saline aqueous fraction removed from the reverse osmosis unit byduct 24,

a fraction of the nanofiltration retentate,

the mother liquor from crystallization unit 12,

raffinates from chromatography units 13 and 15.

It will be specified that according to the monovalent Na⁺, K⁺ and Cl⁻cations and anions content of these different liquids usable for theregeneration of the resins of columns 2 and 4, this regeneration will beable to be performed either in series or in parallel.

It will be noted however that the regeneration in parallel is especiallypreferred because it prevents all risk of precipitation of insolublesalts, such as the calcium phosphate, on the strong cationic resinpresent in column 4.

However, the regeneration in series of both resins is possible on thecondition that the pH is controlled in order to prevent any risk ofprecipitation on the resins.

Moreover, the table hereafter shows the influence of the type ofdecalcification prior to the nanofiltration on the performance of thisnanofiltration, the liquid treated being a permeate resulting from theultrafiltration of a whey (designated whey permeate in this table), thenanofiltration concentration factor being of 4 and the nanofiltrationmembrane being of the type DESAL 5, from the American company OSMONICS.

TABLE Nanofiltration retentate Whey AF then permeate Control CF CF Drymatter (g/l) 50.0 187 187 187 total cations 1.7 1.22 1.14 0.90 (eq./kgof dry matter) total reduction 28 33 47 rate of the cations (%) Control:total absence of decalcification before the nanofiltration CFdecalcification by passage through a strong cationic resin (SR1 LNA fromAmerican company Rohm and Haas). AF then CF: decalcification by serialpassage through a strong anionic resin (IRA 458 from American companyRohm and Haas) then through a strong cationic resin.

The data contained in this table show that the total reduction rate ofthe cations is increased when only the CF system is used, andparticularly increased when the AF-CF system is used; this tabletherefore shows the strong influence on the performances of thenanofiltration of a prior reduction of the content in multivalentcations, in multivalent inorganic anions and in organic acid anionssuited to form complexes with said multivalent cations.

1. Preparation method of crystallized sugar from an aqueous solutioncontaining one or several sugars, multivalent cations, monovalent metalcations, monovalent anions and multivalent inorganic anions and/ororganic acid anions, characterized in that it comprises operations: (a)of replacement of at least a part of said multivalent cations and/or ofsaid multivalent inorganic anions and organic acid anions by monovalentmetal cations and monovalent anions, respectively, in order to obtain anaqueous solution depleted in multivalent cations and/or multivalentinorganic anions and organic acid anions, and containing the saidmonovalent metal cations and monovalent anions, (b) of nanofiltration ofthe solution resulting from operation (a) in order to obtain as aretentate, a sugar aqueous juice enriched in sugars, in multivalentcations and in multivalent inorganic anions and/or in organic acidanions, and as a permeate, an aqueous effluent enriched in monovalentanions and monovalent metal cations, and (c) of crystallization of atleast a part of the retentate resulting from operation (b), in order toobtain crystallized sugar and a mother liquor enriched in monovalentanions and monovalent metal cations.
 2. Method according to claim 1,characterized in that the replacement operation of the multivalentcations is performed simultaneously to the replacement operation of themultivalent inorganic anions and/or organic acid anions, or performed onthe aqueous solution having undergone beforehand the replacementoperation of the multivalent inorganic anions and/or organic acidanions.
 3. Method according to claim 2, characterized in that thereplacement operation (a) comprises the processing of the aqueoussolution with a cationic resin of which the counter-ion is a monovalentmetal cation and/or with an anionic resin of which the counter-ion is amonovalent anion.
 4. Method according to claim 3, characterized in thatit further comprises an operation: (d) of regeneration of the cationicresin and/or of the anionic resin.
 5. Method according to claim 1,characterized in that the replacement operation of the multivalentinorganic anions and/or organic acid anions is performed simultaneouslyto the replacement operation of the multivalent cations or performed onthe aqueous solution having undergone beforehand the replacementoperation of the multivalent cations.
 6. Method according to claim 5,characterized in that the replacement operation (a) comprises theprocessing of the aqueous solution with a cationic resin of which thecounter-ion is a monovalent metal cation and/or with an anionic resin ofwhich the counter-ion is a monovalent anion.
 7. Method according toclaim 6, characterized in that the monovalent metal cation forming thecounter-ion of the cationic resin and the monovalent anion forming thecounter-ion of the anionic resin are of the same type as said monovalentmetal cations and said monovalent anions present in the initial aqueoussolution, respectively.
 8. Method according to claim 7, characterized inthat it further comprises an operation: (d) of regeneration of thecationic resin and/or of the anionic resin.
 9. Method according to claim8, characterized in that regeneration operation (d) comprises theprocessing of the cationic resin and/or of the anionic resin withpermeate obtained during nanofiltration operation (b), after itsconcentration to the desired degree.
 10. Method according to claim 9,characterized in that it further comprises the operation: (e) ofchromatography of at least a part of the mother liquor produced bycrystallization operation (c), in order to obtain an effluent enrichedin sugar and a raffinate enriched in monovalent anions and monovalentmetal cations.
 11. Method according to claim 10, characterized in thatit further comprises the operation: (f) of chromatography of a part ofthe retentate resulting from operation (b), in order to obtain aneffluent enriched in sugar and a raffinate enriched in monovalent anionsand monovalent metal cations.
 12. Method according to claim 11,characterized in that it further comprises the operation: (g) ofprocessing of the permeate resulting from operation (b), by reverseosmosis or electrodialysis in order to produce water and an aqueousfraction enriched in monovalent anions and monovalent metal cations. 13.Method according to claim 12, characterized in that it further comprisesthe operation: of regeneration of the cationic resin and/or of theanionic resin by processing the same with at least one of the followingliquids, possibly concentrated, combined to a part of the permeateobtained during operation (b): mother liquor obtained during operation(c), raffinate obtained during operation (e), raffinate obtained duringoperation (f), aqueous fraction obtained during operation (g). 14.Method according to claim 1, for the purification of a whey, of apermeate resulting from the ultrafiltration of a whey or of a juice ofsugar beetroot, sugarcane, chicory or Jerusalem Artichokes, this whey,permeate or juice comprising Ca²⁺ and Mg²⁺ ions, Cl⁻ anions, Na⁺ or K⁺cations and anions selected mainly in the group consisting of phosphateand sulfate anions, anions from organic acids and their mixtures. 15.Method according to claim 1, characterized in that the replacementoperation (a) comprises the processing of the aqueous solution with acationic resin of which the counter-ion is a monovalent metal cationand/or with an anionic resin of which the counter-ion is a monovalentanion.
 16. Method according to claim 15, characterized in that themonovalent metal cation forming the counter-ion of the cationic resinand the monovalent anion forming the counter-ion of the anionic resinare of the same type as said monovalent metal cations and saidmonovalent anions present in the initial aqueous solution, respectively.17. Method according to claim 16, characterized in that it furthercomprises an operation: (d) of regeneration of the cationic resin and/orof the anionic resin.
 18. Method according to claim 17, characterized inthat regeneration operation (d) comprises the processing of the cationicresin and/or of the anionic resin with permeate obtained duringnanofiltration operation (b), after its concentration to the desireddegree.
 19. Method according to claim 1, characterized in that itfurther comprises the operation: (e) of chromatography of at least apart of the mother liquor produced by crystallization operation (c), inorder to obtain an effluent enriched in sugar and a raffinate enrichedin monovalent anions and monovalent metal cations.
 20. Method accordingto claim 1, characterized in that it further comprises the operation:(f) of chromatography of a part of the retentate resulting fromoperation (b), in order to obtain an effluent enriched in sugar and araffinate enriched in monovalent anions and monovalent metal cations.